Local mating competition, but not climate, drives male reproductive success across a latitudinal gradient in a nest-brooding marine fish

Understanding ecological drivers of reproductive success is crucial to predict whether natural populations can cope with the pace of anthropogenically driven environmental change. In marine ecosystems, this knowledge is difficult to acquire due to the lack of tractable field systems. Here, we took advantage of the nest-brooding behavior of the two-spotted goby Pomatoschistus flavescens, an important planktivorous fish in Scandinavian coastal ecosystems, to study its reproduction across the steep climatic gradient of its natural range. We deployed 360 artificial nests in the field, covering six populations during the breeding season of 2022. We found that climate explained differences among populations in the phenotypes of nest-holding males, and in the impact of both marine growth and parental cannibalism on the broods. In addition, climate affected egg density and diameter. Despite these ecological effects, and although populations differed in average male reproductive success, reproductive success was not influenced by climate. Instead, it was largely determined by competition occurring at the local scale, in particular through the acquisition of high-quality nests, which was itself affected by the relative size of males within the local pool. We propose that the frequency-dependent nature of mating competition may buffer reproductive success against climatic influence in P. flavescens, and discuss the potential generality of such mechanisms and implications for population resilience.